Turbines are used in a variety of aviation, industrial and power generation applications. Typically, gas turbines operating under relatively high pressure and relatively high temperature conditions, include a plurality of rotating turbine blades extending from a rotor. These turbine blades may be driven by one or more hot gases. Any leakage of the hot gas around one or more of the rotating turbine blade tips may reduce the efficiency of the turbine. Thus, the turbine is typically provided with a shroud assembly to minimize a significant leakage of the hot gas. The shroud assembly is typically fixed to a turbine casing and covers the rotating turbine blades. In this regard, the shroud assembly typically provides a circumferential covering to the rotating turbine blades. The gas turbines that include shroud assemblies may provide the advantage of minimum hot gas leakage and, therefore, improve the turbine efficiency.
Conventionally, the shroud assembly of a turbine has an outer shroud and a plurality of inner shrouds. The outer shroud is typically secured to the turbine casing or shell. A typical inner shroud may include an upper surface, a lower surface, a first (forward) end portion and a second (aft) end portion. The lower surface of the inner shroud is typically placed adjacent to the rotating turbine blades. The use of the shroud assembly in the turbine may prevent or minimize the leakage of hot gases into the secondary flow path and may reduce the vibration of the blade tip for each of the rotating turbine blades. Additionally, as each of the plurality of inner shrouds is continuously in contact with the hot gas, the upper surface of each of the inner shrouds is typically covered with an impingement cooling plate for cooling each of the inner shrouds.
Under typical operating and load conditions, the plurality of rotating turbine blades rotate with a fixed number of revolutions per minute. The rotation of the plurality of turbine blades typically causes excitation and vibration of one or more of the plurality of rotating turbine blades with an excitation frequency. Besides, the inner shroud has a harmonic frequency and a plurality of modal vibration frequencies of vibration. The harmonic frequency and the plurality of modal vibration frequencies of the inner shroud are typically a function of its mass and design or structural features, for example, the thickness of a plurality of rails extending between the first end portion and the second end portion or the thickness of the impingement cooling area. A concern arises when at least one of the modal vibration frequencies of the inner shroud lies close to the excitation frequency of one or more of the rotating turbine blades. Such a situation may result in resonance or modal excitation in the inner shroud. This resonance may cause the seal that separates the secondary flow path from the hot gas path to crack, leading to a leakage of the hot gas to the secondary flow path, and thereby reducing the efficiency of the turbine. Additionally, hot gas path (HGP) ingestion may occur and reduce the cooling to the outer shroud. Thus, the temperature of the outer shroud may also increase, increasing the risk of structural damage to the outer shroud. The leakage of the hot gas may, therefore, reduce the life cycle of the shroud assembly and increase the maintenance and repair cost associated with the shroud assembly. Additionally, the leakage of the hot gas may adversely affect the performance of the turbine.
Accordingly, there is a need for an improved inner turbine shroud design that assists in modifying the vibration within the inner turbine shroud.